AH Chemistry – Unit 1

Shapes of Molecules and Polyatomic Ions

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Introduction

This topic explores further aspects of Covalent Bonding, leading to an understanding of the Shapes of molecules & polyatomic ions

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Covalent BondingA Covalent Bond will form when atoms can rearrange their electrons (by sharing) to produce an arrangement of lower energy.

Bond Enthalpy

Bond Length

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Ionic BondingAn Ionic Bond will form when atoms can rearrange their electrons (transfer) to produce an arrangement of lower energy.

Despite the advantage gained by achieving a ‘stable octet’, the real driving force behind ionic bonding is the stability of the Ionic Lattice formed.

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Electronegativity 1Ionic and Covalent are, in fact, rather arbitrary labels and it is no longer enough to simply look to see if a metal element is involved or not.

Electronegativity values are a useful guide but properties will still need to be studied to provide confirmation.

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Electronegativity 2

Ionic and Covalent are just opposite ends of a bonding continuum

Most bonds lie between these two extremes.

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Electronegativity 3

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Bonding Diagrams

Various methods (models) can be used to explain the properties associated with covalent bonding.

One useful model positions electrons ( both bonding and non-bonding) around atoms at ‘four corners’. These are sometimes called Lewis Diagrams

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Bonding StructuresIn Ammonia the central nitrogen atom has 4 electron pairs, the ‘stable octet’

There are 3 ‘bonding pairs’ and 1 ‘non-bonding pair’ - often referred to as a ‘lone pair’

Sometimes atoms will use a lone pair as a bonding pair to form a Dative or Coordinate Covalent bond, as in the Ammonium ion, NH4

+

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More Bonding Structures

C C

H H

H H

HH

.. ...... ... ...

Ethane

C Cl

Cl

Cl

Cl....

........ ..

...... ..

CCl4 Carbon Tetrachloride

..

..C O ..

CO Carbon Monoxide

.. CO O......

CO2 Carbon Dioxide

HO

H.. ..

H2O WaterHydrogen Oxide

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Unusual BondingIn both these molecules, the Be and B atoms do not achieve a ‘stable octet’. The resulting molecules are, predictably, very reactive (unstable).

In both these molecules, the P and S atoms have in excess of a ‘stable octet’.

The extra orbitals needed, to cope with more than 8 electrons, come from the empty d-orbital set.

This ‘mixing’ of orbitals is called hybridisation

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Shapes of MoleculesIn order to predict molecular shape, we consider that all outer shell electrons (valence electrons) of the central atom repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion.Both bonding and non-bonding electron pairs must be considered.

We call this process the Valence Shell Electron Pair Repulsion (VSEPR) theory.

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Electron Pair Geometry 1

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Electron Pair Geometry 2

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Counting Electron PairsIt is important not to confuse the number of atoms with the number of electron pairs.

For example, CF4 and XeF4 may, at first sight, appear likely to be the same shape.

Electron pairs = electrons of central atom + no. of atoms attached

divided by 2CF4 = 4 + 4

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XeF4 = 8 + 4

2

= 4 electron pairs

= 6 electron pairs

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Molecular Geometry 1

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Molecular Geometry 2

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Molecular Geometry 3

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Molecular Geometry 4

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Repulsive Forces 1Bonding electrons, because they are attracted by two nuclei, do not repel as much as non-bonding electrons.

This can cause ‘distortions’ in the shapes of molecules:

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Repulsive Forces 2Electron pair repulsions decrease in strength in the order:

non-bonding/non-bonding

non-bonding/bonding

bonding/bonding

? ? ?

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Repulsive Forces 3Placing the non-bonding lone pairs at the Axial positions would appear to give least repulsion but they would only be 90° away from the 3 bonding pairs.

To minimize e−− e− repulsion, lone pairs are always placed in equatorial positions, so …